In a general aspect, a method for authenticating a user can include: receiving, from a secure processor, a software component configured to generate an image frame including information intelligible to the user, and input data of the software component, the software component can include a first input for receiving a first input data having two randomly-selected valid values, and invalid values. The transmitted input data can include the valid values of the first input data. The method can also include, performing a plurality of times: selecting one of the valid values of the first input data in the transmitted input data, executing the software component by applying the transmitted input data to inputs of the software component and the selected valid value to the first input of the software component. The execution of the software component generating the image frame can have pixels in a visible or invisible state, depending on the selected valid value and defining the information, displaying the image frame. The method can also include, acquiring, from the user, a response depending on the information in the displayed image frame, and transmitting the acquired response to the secure processor.